III-nitride light emitting device with reduced strain light emitting layer

ABSTRACT

In accordance with embodiments of the invention, strain is reduced in the light emitting layer of a III-nitride device by including a strain-relieved layer in the device. The surface on which the strain-relieved layer is grown is configured such that strain-relieved layer can expand laterally and at least partially relax. In some embodiments of the invention, the strain-relieved layer is grown over a textured semiconductor layer or a mask layer. In some embodiments of the invention, the strain-relieved layer is group of posts of semiconductor material.

BACKGROUND

1. Field of Invention

The present invention relates to growth techniques and device structuresfor semiconductor light emitting devices.

2. Description of Related Art

Semiconductor light-emitting devices including light emitting diodes(LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavitylaser diodes (VCSELs), and edge emitting lasers are among the mostefficient light sources currently available. Materials systems currentlyof interest in the manufacture of high-brightness light emitting devicescapable of operation across the visible spectrum include Group III-Vsemiconductors, particularly binary, ternary, and quaternary alloys ofgallium, aluminum, indium, and nitrogen, also referred to as III-nitridematerials. Typically, III-nitride light emitting devices are fabricatedby epitaxially growing a stack of semiconductor layers of differentcompositions and dopant concentrations on a suitable substrate bymetal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy(MBE), or other epitaxial techniques. The stack often includes one ormore n-type layers doped with, for example, Si, formed over thesubstrate, a light emitting or active region formed over the n-typelayer or layers, and one or more p-type layers doped with, for example,Mg, formed over the active region. III-nitride devices formed onconductive substrates may have the p- and n-contacts formed on oppositesides of the device. Often, III-nitride devices are fabricated oninsulating substrates with both contacts on the same side of the device.

SUMMARY

In embodiments of the invention, a III-nitride light emitting deviceincludes a strain-relieved layer designed to reduce strain in thedevice, in particular in the light emitting layer. Reducing the strainin the light emitting device may improve the performance of the device.The surface on which the strain-relieved layer is grown is configuredsuch that strain-relieved layer grows only on portions of the surface,providing space for the strain-relieved layer to expand laterally and atleast partially relax. In some embodiments of the invention, thestrain-relieved layer is grown over a textured semiconductor layer or amask layer. In some embodiments of the invention, the strain-relievedlayer is group of posts of semiconductor material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a portion of a light emitting device with astrain-relieved light emitting layer grown on a textured layer.

FIG. 2 illustrates a portion of a light emitting device with a lightemitting layer grown over a strain-relieved layer grown on a texturedlayer.

FIG. 3 illustrates a portion of a light emitting device with a lightemitting layer grown over a mask.

FIG. 4 illustrates a portion of a light emitting device with a lightemitting layer grown within a group of posts of semiconductor material.

FIG. 5 illustrates a portion of a light emitting device with a lightemitting layer grown over a coalesced layer grown over a group of postsof semiconductor material.

FIGS. 6 and 7 illustrate portions of light emitting devices with lightemitting layers grown over groups of posts of semiconductor material andwith resistive material electrically isolating regions of n- and p-typematerial.

FIG. 8 illustrates a portion of a flip chip light emitting device fromwhich the growth substrate has been removed.

FIG. 9 is an exploded view of a packaged light emitting device.

FIGS. 10 and 11 illustrate portions of light emitting devices withconformal light emitting layers grown over polyhedrons grown overopenings in a mask.

DETAILED DESCRIPTION

The performance of a semiconductor light emitting device may be gaugedby measuring the internal quantum efficiency, which measures the numberof photons generated in the device per electron supplied to the device.As the current density applied to a conventional III-nitride lightemitting device increases, the internal quantum efficiency of the deviceinitially increases, then decreases. As the current density increasespast zero, the internal quantum efficiency increases, reaching a peak ata given current density (for example, at about 10 A/cm² for somedevices). As current density increases beyond the peak, the internalquantum efficiency initially drops quickly, then the decrease slows athigher current density (for example, beyond 200 A/cm² for some devices).

One technique to reduce or reverse the drop in quantum efficiency athigh current density is to form thicker light emitting layers. Forexample, a light emitting layer configured to emit light at 450 nm ispreferably thicker than 50 Å. The charge carrier density in a thickerlight emitting layer may be less than the charge carrier density in aquantum well, which may reduce the number of carriers lost tononradiative recombination and thereby increase the external quantumefficiency. However, growth of thick III-nitride light emitting layersis difficult because of the strain in III-nitride device layers.

Since native III-nitride growth substrates are generally expensive, notwidely available, and impractical for growth of commercial devices,III-nitride devices are often grown on sapphire or SiC substrates. Suchnon-native substrates have different lattice constants than the bulklattice constants of the III-nitride device layers grown on thesubstrate, resulting in strain in the III-nitride layers grown on thesubstrate. As used herein, an “in-plane” lattice constant refers to theactual lattice constant of a layer within the device, and a “bulk”lattice constant refers to the lattice constant of relaxed,free-standing material of a given composition. The amount of strain in alayer is the difference between the in-plane lattice constant of thematerial forming a particular layer and the bulk lattice constant of thelayer in the device, divided by the bulk lattice constant of the layer.

When a III-nitride device is conventionally grown on Al₂O₃, the firstlayer grown on the substrate is generally a GaN buffer layer with anin-plane a-lattice constant of about 3.1885 Å. The GaN buffer layerserves as a lattice constant template for the light emitting region inthat it sets the lattice constant for all of the device layers grownover the buffer layer, including the InGaN light emitting layer. Sincethe bulk lattice constant of InGaN is larger than the in-plane latticeconstant of the GaN buffer layer template, the light emitting layer isstrained when grown over a GaN buffer layer. For example, a lightemitting layer configured to emit light of about 450 nm may have acomposition In_(0.16)Ga_(0.84)N, a composition with a bulk latticeconstant of 3.242 Å. As the InN composition in the light emitting layerincreases, as in devices emitting light at longer wavelengths, thestrain in the light emitting layer also increases.

If the thickness of the strained layer increases beyond a criticalvalue, dislocations or other defects form within the layer to reduce theenergy associated with the strain. The defects become nonradiativerecombination centers which can considerably reduce the quantumefficiency of the device. As a result, the thickness of the lightemitting layer must be kept below this critical thickness. As the InNcomposition and peak wavelength increase, the strain in the lightemitting layer increases, thus the critical thickness of a lightemitting layer decreases.

Even if the thickness of the light emitting layer is kept below thecritical thickness, InGaN alloys are thermodynamically unstable atcertain compositions and temperatures. For example, at temperaturestypically used for InGaN growth, the alloy may exhibit spinodaldecomposition, where a compositionally uniform InGaN layer transformsinto a layer with regions of higher-than-average InN composition andregions of lower-than-average InN composition. Spinodal decomposition inan InGaN light emitting layer creates nonradiative recombination centerswhich may reduce the quantum efficiency of the device. The problem ofspinodal decomposition worsens as the thickness of the light emittinglayer increases, as the average InN composition in the light emittinglayer increases, and/or as the strain in the light emitting layerincreases. For example, in the case of a light emitting layer grown overa sapphire substrate and configured to emit light at 450 nm, thecombination of an InN composition of 16% and the preferred thickness ofgreater than 50 Å exceeds the spinodal decomposition limit.

Accordingly, as described above, it is desirable to increase thethickness of the light emitting layer to reduce or eliminate the drop inquantum efficiency that occurs as the current density increases. It isnecessary to reduce the strain in the light emitting layer in order togrow a thicker light emitting layer, to keep the number of defectswithin an acceptable range by increasing the critical thickness, and toincrease the thickness at which layer can be grown without spinodaldecomposition. Embodiments of the invention are designed to reducestrain in the device layers of a III-nitride device, in particular inthe light emitting layer.

In accordance with embodiments of the invention, at least partial strainrelief in a light emitting layer of a III-nitride light emitting deviceis provided by configuring the surface on which at least one layer ofthe device grows such that the layer expands laterally and thus at leastpartially relaxes. This layer is referred to as the strain-relievedlayer. In a conventional device, all the layers in the device are grownthin enough that they are strained, thus the first single crystal layergrown over the growth substrate sets the lattice constant for eachstrained layer in the device. In embodiments of the invention, thestrain-relieved layer at least partially relaxes, such that the latticeconstant in the strain-relieved layer is larger than the latticeconstant of the layer grown before the strain-relieved layer. Thestrain-relieved layer thus expands the lattice constant for the layersgrown subsequent to the strain-relieved layer.

In some embodiments, the light emitting layer itself is thestrain-relieved layer, meaning that the light emitting layer is grown ona surface that allows the light emitting layer to expand laterally torelieve strain. In some embodiments, a layer grown before the lightemitting layer is the strain-relieved layer. In a first group ofembodiments, the strain-relieved layer is grown on a textured surface.In a second group of embodiments, the strain-relieved layer is grownwithin or over posts of III-nitride material, often referred to asnanowires or nanocolumns.

In the embodiments described below, the III-nitride light emittingdevice includes an n-type region typically grown first over a suitablegrowth substrate. The n-type region may include multiple layers ofdifferent compositions and dopant concentration including, for example,preparation layers such as buffer layers or nucleation layers which maybe n-type or not intentionally doped, release layers designed tofacilitate later release of the growth substrate or thinning of thesemiconductor structure after substrate removal, and n-type devicelayers designed for particular optical or electrical propertiesdesirable for the light emitting region to efficiently emit light.

A light emitting region is grown over the n-type region. Though theembodiments below may refer to a single light emitting layer, it is tobe understood that any of the embodiments below may include a lightemitting region with one or more thick or thin light emitting layers.Examples of suitable light emitting regions include a single thick orthin light emitting layer and a multiple quantum well light emittingregion including multiple thin or thick quantum well light emittinglayers separated by barrier layers.

In some embodiments, the thickness of each of the light emitting layersin the device is preferably thicker than 50 Å. In some embodiments, thelight emitting region of the device is a single, thick light emittinglayer with a thickness between 50 and 600 Å, more preferably between 100and 250 Å. The optimal thickness may depend on the number of defectswithin the light emitting layer. The concentration of defects in thelight emitting region is preferably limited to less than 10⁹ cm⁻², morepreferably limited to less than 10⁸ cm⁻², more preferably limited toless than 10⁷ cm⁻², and more preferably limited to less than 10⁶ cm⁻².

In some embodiments, at least one light emitting layer in the device isdoped with a dopant such as Si to a dopant concentration between 1×10¹⁸cm⁻³ and 1×10²⁰ cm⁻³. Si doping may influence the in-plane a latticeconstant in the light emitting layer, potentially further reducing thestrain in the light emitting layer.

A p-type region is grown over the light emitting region. Like the n-typeregion, the p-type region may include multiple layers of differentcomposition, thickness, and dopant concentration, including layers thatare not intentionally doped, or n-type layers.

FIG. 1 illustrates an embodiment of the invention where astrain-relieved light emitting layer is grown over the textured surfaceof a semiconductor layer. In the device of FIG. 1, an n-type region 11having an in-plane lattice constant a₁ is grown over a growth substrate20. The top surface of n-type region 11, which may be, for example, GaN,InGaN, AlGaN, or AlInGaN, is textured. A strain-relieved light emittinglayer 12 having an in-plane lattice constant a₂ is then grown over thetextured surface. A p-type region 13, which also has an in-plane latticeconstant a₂, is grown over light emitting layer 12.

The surface of n-type region 11 is textured with a controlled, roughsurface, such as, for example, with features having a cross sectionalprofile of peaks alternating with valleys. The distance between adjacentpeaks may be 50 to 200 nm, more preferably 50 to 100 nm. The depth fromthe top of a peak to the bottom of a valley may be less than 200 nm,more preferably less than 100 nm. Features of appropriate size, depth,and spacing may be formed by, for example, conventionalphotolithographic etching, sputter etching, photoelectrochemicaletching, or by an in situ process wherein the crystalline material isgrown textured, such as by growth at elevated pressure. When thefeatures are appropriately sized, the InGaN material of light emittinglayer 12 preferentially grows on the peaks as a group of islands. Sinceinitially the islands do not cover the entire surface of textured n-typeregion 11, the islands may expand laterally such that light emittinglayer 12 at least partially relaxes. The in-plane lattice constant a₂ ofstrain-relieved light emitting layer 12 is larger than the in-planelattice constant a₁ of n-type region 11.

FIG. 2 illustrates a variation of the device of FIG. 1, where the layergrown on the textured surface in order to provide strain relief is notlight emitting layer 12, rather it is an n-type layer 21 grown overn-type region 11 before light emitting layer 12. As in the device ofFIG. 1, an n-type region 11 having an in-plane lattice constant a₁ isgrown over a growth substrate 20. The top surface of n-type region 11 istextured as described in reference to FIG. 1. A second n-type region 21,which may be GaN, InGaN, AlGaN, or AlInGaN, is grown over the texturedsurface of n-type region 11. As n-type region 21 begins to grow, theIII-nitride material of n-type region 21 preferentially grows on thepeaks of the textured surface of n-type region 11 as a group of islands.The islands of material may expand laterally and at least partiallyrelax, such that the in-plane lattice constant a₂ of n-type region 21 islarger than the in-plane lattice constant a₁ of n-type region 11. Thelayers grown over strain-relieved region 21, including light emittinglayer 12 and p-type region 13, replicate the larger in-plane latticeconstant a₂ of strain-relieved region 21.

FIG. 3 illustrates an embodiment of the invention where astrain-relieved layer is grown over a mask. In the device of FIG. 3, ann-type region 14 having a lattice constant a₁ is grown over a growthsubstrate 20. The surface of n-type region 14 is treated with a siliconprecursor such as silane such that the surface is partially covered withsilicon nitride material SiN_(x) and partially exposed in small openingsin the silicon nitride, creating a mask. The exposed regions may have alateral extent of 10 to 200 nm, more preferably 50 to 150 nm, and morepreferably smaller than 100 nm.

A light emitting region 17 is grown over the mask. The material of lightemitting region 17 preferentially grows on the openings 16 in maskmaterial 15, on the exposed the surface of n-type region 14. The islandsof light emitting layer material can expand laterally and at leastpartially relax, such that the in-plane lattice constant a₂ of lightemitting region 17 is larger than the in-plane lattice constant a₁ ofn-type region 14. A p-type region 18, also having an in-plane latticeconstant a₂, is grown over light emitting region 17. As in the devicesshown in FIGS. 1 and 2, light emitting region 17 need not be growndirectly over the mask, rather a second n-type region of, for exampleGaN, InGaN, AlGaN, or AlInGaN, may be grown first on the mask, followedby light emitting region 17.

In the embodiments illustrated in FIGS. 1, 2, and 3, where the lightemitting layer is grown over a textured interface such as the texturedlayers in FIGS. 1 and 2 or the mask layer in FIG. 3, the texturedinterface is generally located close to the light emitting layer. Insome embodiments, the textured interface is within 1000 Å of at least aportion of the light emitting layer.

FIGS. 4, 5, 6, and 7 illustrate devices including posts of semiconductormaterial. In FIG. 4, an n-type region 22 is grown over a substrate 20.Over planar n-type region 22, a mask layer 24 such as the SiN_(x) maskdescribed above is formed. In the openings between islands of maskmaterial, posts of semiconductor material are grown. In someembodiments, the growth temperature of the posts of semiconductormaterial is kept below a temperature at which the GaN material betweenthe islands of masked material begins to decompose, 1000° C. in someapplications. The posts of semiconductor material may be grown within amore narrow temperature range than a planar layer grown over a mask, asin FIG. 3, and under conditions that favor slow growth, in order to formposts of semiconductor material rather than the substantially planarlayer of FIG. 3. For example, posts may be grown at a growth temperaturebetween 900 and 1000° C., at a growth rate less than 0.5 Å/s, and at aratio of group V precursors to group III precursors greater than 4000.Planar material may be grown at temperatures greater than 1000° C. andless than 900° C., at faster growth rates, and at different precursorratios. Posts 26 of n-type material are grown first, followed by posts28 of light emitting region material, followed by posts 30 of p-typematerial.

After p-type posts 30 are grown, the growth conditions are changed, forexample by introducing or increasing the flow of a dopant precursor suchas a Mg-dopant precursor, by decreasing the flow of nitrogen precursor(generally NH₃), and by increasing the growth rate, such that invertedpyramids are formed over the posts, which pyramids eventually coalesceto form a planar layer 32 over the posts and spaces 25 between theposts.

The dimensions of the posts of III-nitride material are selected suchthat the posts may expand laterally to accommodate the difference inlattice constant between layers of different composition within theposts. For example, the diameter of the posts may be limited to lessthan 500 nm, more preferably less than 200 nm. Diameters as small as 10nm may be possible. Diameters between 50 and 150 nm, for example in areaof 100 nm, are likely. The diameter is selected to be small enough suchthat the material in the posts can at least partially relax, and largeenough that there is an acceptably high fill factor of light emittinglayer material. The posts need not have a constant diameter, asillustrated in FIG. 4. For example, the posts may be truncated pyramids.In some embodiments, the fill factor is at least 90%, meaning that asgrown, the posts occupy at least 90% of the lateral extent of thesemiconductor structure of the device. The fill factor is determined byboth the diameter of the posts and the spacing between the posts. If thediameter of the posts is reduced, the number density of posts mustincrease to maintain a given fill factor. In some embodiments, thenumber density of posts is at least 10¹⁰ cm⁻².

The height of the posts may range from 50 nm to 3 μm. In a device with asingle light emitting layer, heights between 50 and 150 nm, for exampleof 100 nm, are likely. In a device with a multiple quantum well lightemitting region, heights between 200 nm and 1 μm, for example of 500 nm,are likely. Light emitting region 28 within the posts may be at leastpartially relaxed.

In some embodiments, in the device illustrated in FIG. 4, the lightemitting regions in different posts in a single device may be formed toemit different wavelengths of light. For example, some of the posts inthe device may be configured to emit reddish light, some of the posts inthe device may be configured to emit greenish light, and some of theposts in the device may be configured to emit bluish light, such thatthe combined red, green, and blue light appears white.

The emission wavelength of a light emitting regions depends on the InNcomposition: the more InN in an InGaN light emitting layer, the longerthe emission wavelength. In conventional devices with planar,uninterrupted light emitting layers, the strain in the light emittinglayer limits the amount of InN that may be incorporated into the lightemitting layer. In general, planar InGaN light emitting layers that emitblue light may be grown at higher quality than planar InGaN lightemitting layers that emit green light. It is extremely difficult to growa planar InGaN light emitting layer of high enough quality that emitslight at a longer wavelength than green. Since a light emitting regiongrown within a post as illustrated in FIG. 4 may at least partiallyrelax, more InN may be incorporated during growth than in a conventionalstrained planar layer. The more relaxed the material in the post, themore InN may be incorporated in the light emitting layer.

The inventors have grown structures with posts including at least oneInGaN layer. The structures were characterized by photoluminescence,which showed the emission wavelength from the InGaN material wassignificantly red-shifted from conventional planar growth. Emissionwavelengths between 430 nm and 750 nm, representing colors from blue tored including green and yellow, have been achieved.

In some embodiments, the InN composition in individual posts iscontrolled by controlling the diameter of the posts. The smaller thediameter of a post, the more relaxed the material in the post, thus themore InN is incorporated during growth of the light emitting region. Forexample, in a device with posts varying in diameter from about 10 nm toabout 150 nm, the posts with diameters in the range of 10 nm areexpected to be the most relaxed, have light emitting regions with thehighest InN compositions, and emit the longest wavelength, most redlight. The posts with diameters in the range of 150 nm are expected tobe less relaxed, have light emitting regions with lower InNcompositions, and emit shorter wavelength, more blue light.

In order to make a device that emits white light, there must be acontrolled number of posts emitting light in each region of the visiblespectrum. As described above, the wavelength of light emitted by eachpost may be controlled by controlling the diameter of the post. Toensure that there are sufficient numbers of each post of a givendiameter and corresponding emission wavelength, mask layer 24 may bepatterned, for example by a nano-imprinting lithography technique, toform a plurality of openings with the desired diameters. Though a deviceemitting white light is used as an example, it is to be understood thatthe emission spectrum from the device can be tailored to other colors oflight by patterning mask 24 with openings of the appropriate size.

A device where different posts emit different colors of light such thatthe combined light appears white may offer benefits over a conventionalwhite-light device, where a blue-emitting semiconductor light emittingdevice is combined with one or more wavelength converting materials suchas phosphors such that the phosphor-converted light combines withunconverted blue light leaking through the phosphor to form white light.A device with posts emitting different colors of light may reducemanufacturing complexity, since it does not require forming wavelengthconverting layers after forming the device; may offer improved controlof chromaticity, color temperature, and color rendering, since theemission spectrum is potentially more easily controlled; may be moreefficient, for example by eliminating inefficiencies associated withwavelength converting materials; may be less expensive to manufacture,since expensive wavelength converting materials are no longer required;and may offer greater flexibility in tailoring the emission spectrum.

In the device of FIG. 5, a strain-reduced light emitting layer is grownover a layer coalesced over a group of semiconductor posts. An n-typeregion 22 having an in-plane lattice constant a₁ is grown over asubstrate 20. Over the planar n-type region 22, a mask layer 24 such asthe SiN_(x) mask described above is formed. In the openings betweenislands of mask material, posts of n-type material 26 are grown. Theposts are grown such that the diameter is small enough that the postsmay expand laterally and thus at least partially relax, as describedabove. When growth conditions are altered such that an n-type region 34coalesces over posts 26, n-type region 34 retains the in-plane latticeconstant of the at least partially relaxed posts and thus has anin-plane lattice constant a₂ which is larger than the in-plane latticeconstant a₁ of n-type region 22. A light emitting region 36 and p-typeregion 38, both of which replicate the in-plane lattice constant a₂, aregrown over n-type region 34.

As n-type region 34 coalesces over posts 26, suture defects 27 may formwhere the material growing over two posts comes together. Defects 27 maybe replicated through light emitting region 36 and p-type region 38 andmay reduce efficiency or cause reliability problems. FIGS. 6 and 7illustrate embodiments of the invention designed to eliminate suturedefects or reduce the number of suture defects.

In the device of FIG. 6, an n-type region 22 is grown over substrate 20,then a mask 24 is formed and n-type posts 26 are grown as describedabove, such that posts 26 at least partially relax. A conformal layer ofresistive material 40 is formed over posts 26. Resistive layer 40 maybe, for example, epitaxially grown resistive GaN such as GaN doped withZn or Fe, or a resistive oxide such as an oxide of silicon. Theresistive layers formed over the tops of posts 26 are then removed byconventional lithography, such that resistive material 40 remains onlyin the spaces between posts 26. Light emitting regions 42 are then grownas posts over the exposed tops of posts 26, followed by a p-type region44 which coalesces over light emitting regions 42. Resistive regions 40electrically isolate n-type regions 22 and 26 from p-type region 44.

In the device of FIG. 7, an n-type region 22 is grown over substrate 20,then a mask 24 is formed and n-type posts 26 are grown as describedabove, such that posts 26 at least partially relax. A conformal layer ofundoped InGaN 46 is grown over posts 26, then growth conditions areswitched to conditions favoring post growth in order to grow posts ofdoped light emitting region 48 over the tops of the regions of conformallayer 46 over posts 26. A p-type region 52 is then grown which coalescesover light emitting regions 48. Doping of the light emitting regionislands 48 results in a lower breakdown voltage than the undoped InGaNregions 46 between posts 26, thus n-type regions 22 and 26 areelectrically isolated from p-type region 52.

In some embodiments, after growth of light emitting region islands 48,an ion implantation step renders regions 50 between posts 26nonconductive. After implantation, the ion damaged InGaN regions 46 overthe tops of posts 26 may be removed by etching. In such embodiments,light emitting region islands 48 are grown directly over posts 26.

In embodiments illustrated in FIGS. 10 and 11, as in FIG. 4, an n-typeregion 22 is grown over a substrate 20. Over planar n-type region 22, amask layer 24 such as the SiN_(x) mask described above is formed. In theopenings 80 between islands of mask material, polyhedrons 82 ofsemiconductor material are grown. Like the posts shown in FIGS. 4 and 5,since polyhedrons 82 are grown in openings 80 between islands of maskmaterial, polyhedrons 82 are able to expand laterally and are thereforeat least partially relaxed. Polyhedrons 82 thus have a lattice constanta₂ larger than the lattice constant a₁ of planar layer 22. In someembodiments, the diameter of openings 80 may be limited to less than 500nm, more preferably less than 200 nm. Diameters as small as 10 nm may bepossible. Diameters between 50 and 150 nm, for example in area of 100nm, are likely. The diameter of openings 80 is selected to be smallenough such that the material in polyhedrons 82 can at least partiallyrelax. As in FIG. 4, mask 24 may be formed such that the fill factor isat least 90%, meaning that as grown, the bases of polyhedrons 82 occupyat least 90% of the lateral extent of the semiconductor structure of thedevice.

At least one light emitting layer 84 is grown over polyhedrons 82 suchthat the material in light emitting layer 84 replicates the expandedlattice constant a₂ of polyhedrons 82. A p-type region is then grownover light emitting layer 84. In the device illustrated in FIG. 10,p-type region 86 preferentially grows over polyhedrons 82. Growth isstopped before the region between adjacent polyhedrons, covered by mask24, is filled in. A thick metal layer (not shown) may be deposited overthe polyhedrons to form a planar surface. Insulating mask layer 24provides electrical isolation between the metal contacting the p-typematerial and the n-type region of the semiconductor in the regionsbetween openings 80. In the device illustrated in FIG. 11, growth ofp-type region 88 continues until the regions between adjacentpolyhedrons are filled in, resulting in a substantially planar p-typelayer.

The light emitting layers in the embodiments described above may havelarger in-plane a-lattice constants than light emitting layers grown onconventional GaN templates, which typically have in-plane a-latticeconstants no larger than 3.1885 Å. Growth of the light emitting layer asor over a strain-relieved layer may increase the in-plane latticeconstant to greater than 3.189 Å, and may thus sufficiently reduce thestrain in the light emitting layer to permit thicker light emittinglayers to be grown with acceptable defect densities and with reducedspinodal decomposition. In some embodiments, the in-plane a-latticeconstant in the light emitting layer may be increased to at least 3.195Å, more preferably to at least 3.2 Å. For example, an InGaN layer thatemits blue light may have the composition In_(0.12)Ga_(0.88)N, acomposition with a bulk lattice constant of 3.23 Å. The strain in thelight emitting layer is the difference between the in-plane latticeconstant in the light emitting layer (about 3.189 Å for light emittinglayer grown on a conventional GaN buffer layer) and the bulk latticeconstant the light emitting layer, thus strain may be expressed as(a_(in-plane)-a_(bulk))/a_(bulk). In the case of a conventionalIn_(0.12)Ga_(0.88)N layer, the strain is (3.189 Å-3.23 Å)/3.23 Å, about1.23%. If a light emitting layer of the same composition is grownaccording to the embodiments described above, the strain may be reducedor eliminated. In some embodiments of the invention, the strain in thelight emitting layer of a device emitting light between 430 and 480 nmmay be reduced to less than 1%, and more preferably to less than 0.5%.An InGaN layer that emits cyan light may have the compositionIn_(0.16)Ga_(0.84)N, a composition with strain of about 1.7% when grownon a conventional GaN buffer layer. In some embodiments of theinvention, the strain in the light emitting layer of a device emittinglight between 480 and 520 nm may be reduced to less than 1.5%, and morepreferably to less than 1%. An InGaN layer that emits green light mayhave the composition In_(0.2)Ga_(0.8)N, a composition with a freestanding lattice constant of 3.26 Å, resulting in strain of about 2.1%when grown on a conventional GaN buffer layer. In some embodiments ofthe invention, the strain in the light emitting layer of a deviceemitting light between 520 and 560 nm may be reduced to less than 2%,and more preferably to less than 1.5%.

The semiconductor structures illustrated and described above may beincluded in any suitable configuration of a light emitting device, suchas a device with contacts formed on opposite sides of the device or adevice with both contacts formed on the same side of the device. Whenboth contacts are disposed on the same side, the device may be formedeither with transparent contacts and mounted such that light isextracted either through the same side on which the contacts are formed,or with reflective contacts and mounted as a flip chip, where light isextracted from the side opposite the side on which the contacts areformed.

FIG. 8 illustrates a portion of one example of a suitable configuration,a flip chip device from which the growth substrate has been removed. Aportion of p-type region 66 and light emitting region 64 is removed toform a mesa that exposes a portion of n-type region 62. Though one viaexposing n-type region 62 is shown in FIG. 8, it is to be understoodthat multiple vias may be formed in a single device. N- and p-contacts70 and 68 are formed on the exposed parts of n-type region 62 and p-typeregion 66, for example by evaporating or plating. Contacts 68 and 70 maybe electrically isolated from each other by air or a dielectric layer.After contact metals 68 and 70 are formed, a wafer of devices may bediced into individual devices, then each device is flipped relative tothe growth direction and mounted on a mount 73, in which case mount 73may have a lateral extent larger than that of the device. Alternatively,a wafer of devices may be connected to a wafer of mounts, then dicedinto individual devices. Mount 73 may be, for example, semiconductorsuch as Si, metal, or ceramic such as AlN, and may have at least onemetal pad 71 which electrically connects to p-contacts 68 and at leastone metal pad 72 which electrically connects to the n-contacts 70.Interconnects (not shown) such as solder or gold stud bumps, connect thesemiconductor device to mount 73.

After mounting, the growth substrate (not shown) is removed by a processsuitable to the substrate material, such as etching or laser melting. Arigid underfill may be provided between the device and mount 73 beforeor after mounting to support the semiconductor layers and preventcracking during substrate removal. A portion of the semiconductorstructure may be removed by thinning after removing the substrate. Theexposed surface of n-type region 62 may be roughened, for example by anetching process such as photoelectrochemical etching or by a mechanicalprocess such as grinding. Roughening the surface from which light isextracted may improve light extraction from the device. Alternatively, aphotonic crystal structure may be formed in the top surface of n-typeregion 62 exposed by removing the grown substrate. A structure 74 suchas a phosphor layer or secondary optics known in the art such asdichroics or polarizers may be applied to the emitting surface.

FIG. 9 is an exploded view of a packaged light emitting device, asdescribed in more detail in U.S. Pat. No. 6,274,924. A heat-sinking slug100 is placed into an insert-molded leadframe. The insert-moldedleadframe is, for example, a filled plastic material 105 molded around ametal frame 106 that provides an electrical path. Slug 100 may includean optional reflector cup 102. The light emitting device die 104, whichmay be any of the devices described in the embodiments above, is mounteddirectly or indirectly via a thermally conducting submount 103 to slug100. A cover 108, which may be an optical lens, may be added.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. Therefore, it is not intended that the scopeof the invention be limited to the specific embodiments illustrated anddescribed.

1. A device comprising: a mask layer having a plurality of openings; aIll-nitride structure comprising: a plurality of posts of semiconductormaterial corresponding to the openings in the mask layer, wherein theplurality of posts are separated by an insulating material, and whereinin a plane parallel to a surface of the mask layer, a lateral extent ofa cross section of the plurality of posts is at least 90% of a lateralextent of the device; a light emitting layer disposed between an n-typeregion and a p-type region, wherein a major surface of the lightemitting layer is substantially parallel to a surface of the mask layer.2. The device of claim 1 wherein the mask layer comprises silicon andnitrogen.
 3. The device of claim 1 wherein each of the posts has adiameter less than 150 nm.
 4. The device of claim 1 wherein the lightemitting layer is disposed within the posts.
 5. The device of claim 1further comprising a planar layer of p-type semiconductor materialdisposed over the plurality of posts.
 6. The device of claim 1 furthercomprising a planar layer of n-type semiconductor material disposed overthe plurality of posts.
 7. The device of claim 1 wherein the posts havea height between 50 nm and 3 μm.
 8. The device of claim 1 wherein theinsulating material is one of air, a resistive III-nitride material, andan oxide of silicon.
 9. The device of claim 1 wherein the posts aretruncated polyhedrons.
 10. The device of claim 1 wherein the posts arepolyhedrons.
 11. The device of claim 1 wherein the light emitting layeris a conformal layer formed over the posts.
 12. The device of claim 1wherein: the light emitting layer has bulk lattice constant a_(bulk)corresponding to a lattice constant of a free standing material of asame composition as the light emitting layer; the light emitting layerhas an in-plane lattice constant a_(in-plane) corresponding to a latticeconstant of the light emitting layer as grown in the structure; and(a_(in-plane)−a_(bulk))/ a_(bulk) is less than 1%.
 13. The device ofclaim 1 wherein the light emitting layer has a thickness greater than 50angstroms.
 14. The device of claim 1 wherein the light emitting layer isdoped with silicon to a dopant concentration between 1×10¹⁸ cm⁻³ and1×10²⁰ cm⁻³.
 15. The device of claim 1 further comprising: contactselectrically connected to the n-type region and the p-type region; and acover disposed over the III-nitride semiconductor structure.